Processes for the polymerization of vinyl pyrrolidone polymers, with or without other ethylenic monomers copolymerizable therewith, at elevated temperature in the presence of free radical polymerization initiators are known. The resulting polymers have wide commercial acceptance because of their low toxicity and solubility in both organic solvents and water.
One of the problems associated with the batch polymerization of vinyl pyrrolidone containing polymers is in the reduction of residual monomer contained in the polymerization reaction product at the end of the normal polymerization. Residual monomer is unacceptable from an environmental point of view. It also represents an economic loss, since there is substantial cost in removing the monomer. Some of the conventional techniques for removing residual monomer have been through physical processes, e.g., steam or inert gas stripping or additional polymerization through subsequent heat treatments. These processes tend to extend the batch polymerization causing long reaction times as well as leading to product discoloration and changes in product molecular weight distribution and viscosity.
Representative patents illustrating approaches to the removal of residual monomer content in polymerization reaction products are as follows:
U.S. Pat. No. 4,053,696 disclose a process for the manufacture of vinyl pyrrolidone polymers practically free from residual monomer and impurities via a continuous polymerization technique. Polymerization is carried in an organic solvent in the presence of free radical polymerization initiators. These initiators are introduced ab initio, i.e., introduced with the initial charge of reactants or introduced at various points of the reaction.
U.S. Pat. No. 3,459,720 discloses an improved process for the polymerization of N-vinyl lactams in the presence of azo catalysts and hydrogen peroxide. Disclosed in the background portion of the patent are two methods for the polymerization of N-vinyl pyrrolidone. One method utilizes a peroxide catalyst and the other uses an azobis(isobutyronitrile) catalyst. The patentees point out that the peroxide catalysts result in products inferior in terms of color stability, odor and viscosity, while the azo catalysts yield products of superior quality, but often have a high molecular weight, too high for many purposes. The improved batch process for the polymerization of N-vinyl pyrrolidone is carried out by polymerizing the monomer in the presence of an azo catalyst and controlling the molecular weight by addition of hydroperoxide. The azo catalyst and hydroperoxide are advantageously added at the start of the polymerization to avoid any need for supervision.
U.S. Pat. No. 4,520,180 discloses a process for the polymerization of N-vinyl pyrrolidone using t-butylperoxypivalate, preferably in a solvent consisting essentially of water, isopropanol, and secondary butyl alcohol. In the polymerization process, N-vinyl pyrrolidone is continuously added to a kettle containing solvent, an initial charge of N-vinyl pyrrolidone and t-butylperoxypivalate catalyst.
U.S. Pat. No. 4,205,161 discloses a process for reducing the residual level of vinyl chloride monomer and vinyl chloride monomer in polymers and copolymers. The process comprises post-polymerizing the polymerization reaction product until there is a decrease in the autogenous pressure, then releasing the pressure, cooling the copolymer to a temperature of from 10.degree. to 40.degree. C. and introducing a redox catalyst to effect final polymerization.
U.S. Pat. No. 4,529,753 discloses a process for reducing the residual monomer level in emulsion polymerization reaction products. Residual acrylonitrile is reduced in a latex by adding stoichiometric amounts of amine, by adding additional catalysts and comonomer, steam or inert gas stripping or passing a latex through an apparatus that reduces pressure. A process described in the '753 patent utilizes temperature and pressure conditions at which the vapor pressure of water in ambient environment is less than the vapor pressure water in the emulsion and then utilizes a free radical generator to polymerize residual monomer in the emulsion.
U.S. Pat. No. 4,241,203 discloses a process for reducing the monomer content in acrylonitrile containing copolymers by heating the polymerization product to a temperature slightly exceeding 130.degree. C. for a time sufficient to permit polymerization of the residual monomer.